Electrostatic printing apparatus for forming multiple copies



y 1962 F. A. SCHWERTZ 3,045,587

ELECTROSTATIC PRINTING APPARATUS FOR FORMING MULTIPLE COPIES Filed June 6, 1957 2 Sheets-Sheet l Bus/um Fume Sur fink-away Many/x CHM/K2767? PwsavrAr/ov Ti IL.

IN VEN TOR. fiywsem A Sew/A972 ,qrmpwsrs July 24, 1962 F. A. SCHWERTZ ELECTROSTATIC PRINTING APPARATUS FOR FORMING MULTIPLE COPIES 2 Sheets-Sheet 2 Filed June 6, 1957 United States Patent 0 3,045,587 ELECTROSTATIC PRINTING APPARATUS FOR FORMING MULTIPLE COPIES Frederick A. Schwertz, Pittsford, N .Y., assignor to Xerox Corporation, a corporation of New York Filed June 6, 1957, Ser. No. 664,135

6 Claims. (Cl. 101-1) The present invention relates generally to high-speed electrostatic printers and more particularly to printers adapted to record information as latent images on a continuous medium and repeatedly to develop said latent images and to transfer the resultant powder images to a printing strip to form multiple copies thereof.

In the copending applications entitled Electrostatic Recording of Information, Serial No. 623,327, filed November 20, 1956, and now abandoned. High-Speed Electrostatic Alphanumerical Printer, Sen'al No. 664,137, filed June 6, 1957, and now U.S. Patent 2,919,967, issued on January 5, 1960, and Electrostatic Alphanumerical Printer with Image Transfer Mechanism, Serial No. 664,138, filed June 6, 1957, there is described a novel electrostatic image-forming technique for recording and printing alphanumerical information. This electrostatic technique overcomes many of the drawbacks characteristic of known mechanical and optical printers,

The electrostatic image-forming process as disclosed in said copending applications makes possible the transfer of electrostatic images to a recording medium at extremely high speeds, these images being subsequently rendered visual by methods conventional in the xerographic art. Digital computer switching circuits co-acting with the electrostatic image-forming apparatus permit the logical spacial ordering of electrostatic images which owe their origin to information pulses received serially in time.

In the art of xerography in its electrophotographic aspects, an electrostatic charge applied to the surface of a photoconductive insulating layer is selectively dissipated by exposure to a pattern of light and shadow to be recorded, thereby forming on the surface of the insulating layer an electrostatic latent image corresponding to said pattern. An image formed in this fashion may be developed by the deposition of finely divided material in conformity with the charge pattern and thereafter fixed by fusing the powder on the surface of a print to which the powder pattern has been transferred. A detailed description of the xerographic technique of the apparatus involved in developing and fixing pictures may be found in the US. patent to Carlson, No. 2,297,691.

In the apparatus disclosed in the above-noted copending applications as distinguished from xerography in its elec trophotographic aspects, physical symbols or characters rather than light patterns are recorded as electrostatic images. The images are then developed by depositing a finely divided powder or an ink mist, the resulting visible images being made permanent by fusing or drying. The electrostatic images are produced by symbol-shaped electrode elements or an array of electrode pins which are brought in close proximity to an insulating surface, such as a web of dielectric material. The web is electrostatically precharged by an intense electric field to a point somewhat below critical stress value.

Transfer of the configuration of the symbol or character from the shaped electrode or the pin array to the insulating web is effected by the use of a relatively low potential triggering pulse which raises the electric field above the critical stress value to produce a field discharge in the space between the insulating web and the electrode. The discharge action gives rise to the formation of an electrostatic pattern of the symbol on the insulating surface. Electronic switching circuits are associated with the electrostatic apparatus to supply trigger pulses thereto in accordance with information received electrically from a digital computer or other signal source. This process is now known as Tesiprinting (Transfer Electro tatic Image).

In the copending application entitled, Two Color Printing Apparatus, Serial No. 664,136, filed June 6, 1957, there is disclosed a printer which makes use both of the xerographic and the Tesi techniques for recording information as latent images on a photoconductive medium. The xerograp'hic technique is employed to project business forms onto the recording medium, while the Tesi technique serves to enter variable data into the forms. The latent images thus produced are developed and then transferred to a printing strip by means of a transfer roller which is suitably biased.

In the printers disclosed in the above-identified applications, only one copy can be produced from the latent images. Ordinarily the electrostatic image degenerates during the transfer step, for when a corona charge is applied to the transfer sheet the electrostatic transfer fields which are established are sufficient to cause breakdown of the air layer between the transfer sheet and the image bearing surface.

It has been found that such breakdown will occur for only a certain range of spacing. When the image hearing surface and the transfer paper are in virtual contact, the air film therebetween is only a few microns thick and in this situation extremely high fields are required to trigger air breakdown. As the spacing is increased, much smaller electrostatic fields may initiate ion avalanching. It is just this situation which exists during the usual transfer operations. The charge applied to the transfer sheet to effect toner transfer creates a field sufiiciently large to cause air breakdown during the separation of the transfer sheet and the image bearing surface. Thus with conventional arrangements, once transfer is effected the image bearing surface is electrostatically distorted and no longer contains a reusable charge pattern.

In View of the foregoing, it is the principal object of the present invention to provide an electrostatic printer in which latent electrostatic images may be formed on a continuous medium, which images may be repeatedly developed and transferred to form an arbitrary number of duplicate copies.

More particularly, it is an object of the invention to provide an electrostatic printer wherein latent images may be formed by both xerographic and Tesi techniques on a cylinder having a photoconductive layer, the latent images being developed and transferred repeatedly to a paper web.

Also an object of the invention is to provide a printer as above described wherein the xerographic cylinder may be maintained in a standby condition ready to receive information, this being accomplished by offsetting the electrostatic image onto a continuous dielectric coated metal belt by the Tesi method.

Briefly stated, in a printer for producing duplicate copies in accordance with the invention, electrostatic character forming means operate to form latent images on a continuous insulating medium representative of input information. These images are developed to form powder images and are transferred electrostatically to a paper strip. The transfer mechanism includes means to convey the paper strip into engagement with the medium to attract the powder image therefrom and thereafter to separate the strip from the medium. Also provided is a biased conductive element adjacent the strip at the region of separation from the insulating surface to create an electrostatic potential difference below air breakdown level between the strip and the insulating surface which prevents distortion of the charge pattern on the surface and permits redevelopment thereof.

adhere to the charged areas.

For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawing, wherein FIG. 1 illustrates schematically one embodiment of the invention operating on an intermittent duty cycle.

FIG. 2 is another embodiment of the invention which may be maintained in 'a standby condition.

FIG. 3 is still another embodiment of the invention in which data may be entered by the Tesi method on business forms electrostatically impressed on the recording cylinder.

Referring now to FIG. 1, a metal cylinder is provided coated with a layer of photosensitive material 11 and driven at a constant velocity. The layer may be composed of amorphous selenium or photoconductive materials in insulating binders, such as zinc oxide. Positioned about the cylinder 10 in the following order is a corona charging station 12, an optical exposure station 13, a character-forming station 14, a development station and a transfer station generally designated by numeral 16.

The charging station 12 includes corona discharge wires connected to a high voltage source 12:: to create an electrostatic field relative to the metal cylinder extending through the photoconductive layer. The ions produced by the resultant discharge deposit on the surface of the photoconductive insulating layer, thereby sensitizing the layer.

Exposure of the sensitized surface is carried out at the exposure station 13 by optically projecting an illuminated copy of a form to be printed through a slit, the copy being moved at a linear speed corresponding to the linear speed of the cylinder to effect a scanning action. Thus a latent image of the copy is formed xerographically on the sensitized surface which is discharged in accordance with the light and shadow pattern to form a corresponding charge pattern.

Next in the path of movement is the character-forming station 14 constituted by a bank of recording heads 17, each having a row of electrode pins 18 which may be select-ively pulsed to provide a configuration of dots having a pattern representative of the input information. The operation of the character-forming station is so synchronized as to enter the characters at their proper places in the form xerographically impressed on the cylinder. A more detailed description of the operation of the characfer-forming station as it operates coordinately with the slit projector to provide x-erographic and Tesi recording may be found in said copending application entitled, Two Color Printing Apparatus.

The cylinder carrying the latent image now is subjected to the operation of a brush at development station 15 which rotates through a supply of developer particles and then against the surface of the cylinder to cause toner to The brush may be a true fiber type or it may be a magnetically formed brush carrying magnetic particles encased in an insulating sheath of a material having the proper triboelectric relationship to the charged cylinder. Thus powder images are formed on the surface of the cylinder.

The cylinder next passes to the transfer station 16 where a paper web 19 drawn from a supply spool 20 and rewound on a take-up spool 21 is caused to move at a linear speed equal to that of the circumference of the cylinder 10. Transfer of the developed image from the surface of the cylinder takes place as the paper moves over a corona charging device 22 connected to a power supply 22a. The paper is electrostatically charged to cause the powder image to be released from the cylinder and to be attracted to the paper.

The paper then passes over an endless conductive belt 23 which is positioned over grounded rollers 24 and 25. Contact between the paper web and the endless belt is maintained as the paper is separated from the cylinder 10.

Because the capacitance of the transfer sheet to the conductive belt is higher than that to the image bearing surface, the effective field below the transfer sheet is small and air breakdown is suppressed. As separation proceeds, the potential of the transfer sheet is prevented from increasing greatly, and the field across the air film consequently decreases sufficiently to preclude breakdown.

A detailed description of the technique for maintaining the charge condition of the image bearing surface during transfer without distortion may be found in the co-pending application entitled Image Reproduction, Serial No. 630,797, filed December 27, 1956, and issued on September 6, 1960, as U.S. Patent 2,951,443. In effect the primary electrostatic image on the cylinder serves as a virtual master and repeated transfers are made possible to provide a desired number of copies of a given recording.

The paper web then moves into a fixing station 26 wherein th transferred powder image is permanently bonded to the web by means of a suitable heating device.

The cylinder continues to rotate until the desired number of copies is made. During this period no further information is fed into the machine and powder images are reformed after each transfer operation.

After the duplication operations are completed, a retractable brush 27 is brought to bear on the cylinder to clear the surface of all residual powder before the surface is sensitized and a new image recorded thereon. This brush is then withdrawn until-the next recording.

The arrangement shown in FIG. 1 is capable only of operating on an intermittent duty cycle in which the time between intervals is long enough to permit the production of multiple copies. Alternatively, the xerographic cylinder 10 in the arrangement shown in FIG. 2 may be maintained in a standby condition ready to receive information by offsetting the electrostatic image onto a continuous dielectric coated metal belt by the Tesi method. The multiple copy operations may then be carried out on the belt. The number of such belts required would depend of course on the duty cycle of the machine.

In FIG. 2, two such continuous belts 28 are shown. The latent images on the cylinder are formed both xerographically and by electrostatic transfer in th manner described in connection with FIG. 1. The images are transferred electrostatically to one of the belts, the others being retracted from the cylinder. When one belt has a recording transferred thereto, it is then retracted and a second belt moved into engagement in readiness for the next duty cycle.

The image on the belt 28 is developed by a brush 29 and is transferred to a paper Web 30 in the manner described previously so as to avoid impairment of the latent charge. When the desired number of copies has been made, the latent image is erased by a corona discharge element 31 and residual powder is removed by a brush cleaner 32. The belt when it engages the cylinder operates at the same linear speed, but when it is retracted therefrom to carry out the reproduction process it may be operated at a higher speed.

Where alphanumeric information is to be entered into a limited number of business forms, say three or four, the problem is one of selectively recording a number of definite shapes or patterns in a predetermined space and time sequence. as is disclosed in FIG. 1 calling for a photoconductive insulating surface is not required. The business forms, as shown in FIG. 3, are made available as raised metal patterns on cylindrical surfaces 33, 34 and 35. In design these cylindrical surfaces are very similar to conventional letter press plates. These cylinders are maintained at ground potential and may be made to roll at will against a precharged dielectric coated cylinder 36. Precharging is effected at a charging station 37.

When the raised pattern comes into contact with the precharged surface, a certain amount of the surface charge will be neutralized and thus produce an electro- In this instance, optical xerography, such static latent image of the business form. The variable alphanumeric information may be introduced into the selected form by the pin heads 38. If it is desired to use a character drum for the insertion of the data, the dielectric coated metal cylinder 36 must be replaced by a dielectric coated endless belt. Transfer of the latent images to the paper web 37 is effected in the manner previously set forth.

While there has been shown what are considered to be preferred embodiments of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims to cover all such changes and modifications as fall within the true scope of the invention.

What is claimed is:

1. Electrostatic printing apparatus for producing multiple copies comprising a rotary cylinder having an insulating surface for sustaining electrostatic charges, means electrostatically to form latent images on said surface of information to be printed repeatedly, and a plurality of printing stages disposed at spaced positions about said cylinder, each stage including an endless conductive belt having an insulating surface, the belts of said stages being selectively engageable with said cylinder surface to effect transfer of said latent image thereon and being retractable from said cylinder to effect printing operations, means to move a selected one of said belts into engagement with said cylinder to effect transfer of a latent image thereon and thereafter to retract said one belt, developer means associated with each belt to form powder images and a transfer mechanism associated with each belt including means to convey a paper strip into engagement with said belt and thereafter to separate said strip therefrom, and means at the region of separation to produce an electrostatic field below the level of breakdown to prevent distortion of said latent image whereby said belt may be used to form duplicate copies.

2. Electrostatic printing apparatus for producing multiple copies comprising a rotary cylinder having a photoconductive surface for sustaining electrostatic charges, projection means to form latent images of a form on said surface, electrostatic character-forming. means responsive to input information to form latent images on said surface of information to be entered in said form and to be printed repeatedly, and a plurality of printing stages disposed at spaced positions about said cylinder, each stage including an endless conductive belt having an insulating surface, the belts of said stages being selectively engageable with said cylinder surface to effect transfer of said latent image thereon whereby when a given belt is engaged the remaining belts are retracted to effect printing operations, means to move a selected one of said belts into engagement with said cylinder to effect transfer of a latent image thereon and thereafter to retract said one belt, developer means associated with each belt to form powder images and a transfer mechanism associated with each belt including means to convey a paper strip into engagement with said belt and thereafter to separate said strip therefrom, and means at the region of separation to produce an electrostatic field below the level of breakdown to prevent distortion of said latent image whereby the belt may be used to form duplicate copies.

3. Electrostatic printing apparatus for producing multiple copies comprising a rotary cylinder having a surface for maintaining electrostatic charges, means electrostatically to form latent images on said surface of information to be printed repeatedly, and a plurality of printing stages disposed at spaced positions about said cylinder, each stage including an endless conductive belt having an insulating surface, the belts of said stages being selectively engageable with said cylinder surface to effect transfer of said latent image thereon, whereby when a given belt is engaged the other belts are retracted to effect printing operations, means to move a selected one of said belts into engagement with said cylinder to effect transfer of a latent image thereon and thereafter to retract said one belt, developer means associated with each belt to form powder images and a transfer mechanism associated with each belt including means to convey a paper strip into engagement with said belt and thereafter to separate said strip therefrom, means at the region of separation to produce an electrostatic tfield below the level of breakdown to prevent distortion of said latent image whereby said belt may be used to form duplicate copies, means to drive the engaged belt at the same linear speed as said cylinder, and means to drive the retracted belts at a relatively high speed.

4. Electrostatic printing apparatus for producing multiple copies comprising a rotary cylinder having a surface for sustaining electrostatic charges, means elect-rostatically to form latent images on said surface of said cylinder of information to be printed repeatedly, an endless conductive belt having an insulating surface for sustaining electrostatic charges, said belt being engageable with said cylinder surface to effect transfer of said latent images and being retractable to effect printing operations, means to move said belt into engagement with said cylinder to effect said transfer and thereafter to retract said belt, means to develop a powder image of the latent image transferred to said belt, means to convey a paper strip into engagement with said belt to effect transfer of said powder image for forming duplicate copies, and means to cause said belt when engaged with said cylinder to travel at the same linear speed thereof and when retracted to travel at a higher speed for printing operations.

5. Printing apparatus in accordance with claim 4 in which said rotary cylinder includes a surface layer comprising a photoconductive insulating layer.

6. Printing apparatus in accordance with claim 5 in which said means electrostatically to form latent images on said surface of said cylinder comprises means to optically expose said cylinder to a light image and means in addition to form latent electrostatic charge patterns in the absence of light exposure and including the use of an electrode arrangement.

References Cited in the file of this patent UNITED STATES PATENTS 2,703,280 Butterfield et a1. Mar. 1, 1955 2,715,360 Brown Aug. 16, 1955 2,752,271 Walkup et a1. June 26, 1956 2,756,676 Steinhilper July 31, 1956 2,803,177 Lowrie Aug, 20, 1957 2,811,101 Devol Oct. 29, 1957 2,919,170 Epstein Dec. 29, 1959 2,919,967 Schwertz Jan. 5, 1960 2,951,443 Byrne Sept. 6, 1960 FOREIGN PATENTS 755,486 Great Britain Aug. 22, 1956 

